1. Field of the Invention
The present invention is broadly concerned with methods and compositions for reducing the bioavailability of metal contaminants in soils. More particularly, the inventive methods comprise mixing a source of phosphorus and an oxide of manganese with the contaminated soil so as to form insoluble metal phosphates. Preferably, the pH of the soil is then adjusted to, and maintained at, specified levels so as to maintain the metals in a non-available form.
2. Description of the Prior Art
Lead (Pb) is toxic to both humans and animals. Lead is particularly toxic to young children. Lead and lead-containing compounds can be found in all parts of the environment, and there is considerable concern regarding lead as a contaminant. For example, lead and lead-containing compounds have been identified as a major hazardous substance at 47% of the 1,219 Superfund sites currently on the Environmental Protection Agency""s National Priorities List.
Currently available techniques for cleaning lead-contaminated soil generally comprise capping or excavating the contaminated soil. However, these treatments are often ineffective in fully removing the lead (or other metals for that matter) or in reducing their bioavailability. Furthermore, soil excavation followed by replacement of the excavated soil with clean soil requires a source of clean soil and a repository for the contaminated soil. Thus, these currently available treatment procedures for lead-contaminated soils are costly, disruptive, and not sustainable.
Phosphorus (P) alone has also been used to treat soil contaminated with lead. While these methods have some usefulness, they do not achieve sufficient reduction in the bioavailability of lead. Thus, the health risks associated with contaminated soil are not sufficiently reduced to allow prior art phosphorus treatments to gain acceptance.
There is a need for improved methods for decreasing lead and other metal concentrations in contaminated soils which can be carried out in situ.
The present invention is broadly concerned with compositions and methods for reducing the bioavailability of metal contaminants in soil. The methods comprise intimately mixing a source of phosphorus and an oxide of manganese with the contaminated soil so as to reduce the metal bioavailability in the soil. Reference is made herein to treatment of contaminated soil, however, this is intended to include other contaminated compositions in addition to or other than soil which contain undesirable levels of metal contaminants (e.g., wastes, sediments, etc.).
In more detail, the phosphorus source should be mixed with the soil at a level of from about 0.1-5% by weight phosphorus, preferably from about 0.1-1.0% by weight phosphorus, and more preferably from about 0.25-0.5% by weight phosphorus, based upon the total weight of the soil taken as 100% by weight. Preferred phosphorus sources include those selected from the group consisting of phosphate rock, orthophosphoric acid, metal phosphates (e.g., alkali and alkaline earth metal phosphates such as calcium orthophosphates and potassium orthophosphates), ammonium phosphates, ammonium orthophosphates (e.g., monoammonium orthophosphates, diammonium orthophosphates, and ammonium polyphosphate liquids which typically include orthophosphates and condensed phosphates), and various P-containing fertilizing materials such as superphosphates and triple superphosphates.
The oxide of manganese should be mixed with the soil at a level of from about 0.1-5% by weight oxide of manganese, preferably from about 0.1-1.0% by weight oxide of manganese, and more preferably from about 0.25-0.5% by weight oxide of manganese, based upon the total weight of the soil taken as 100% by weight. Preferred oxides of manganese include Mn2O and Mn3O4, as well as those derived from minerals such as birnessite, cryptomelane, psilomelanes, pyrolusite, nsutite, hollandite, coronadite, romanechite, vernadite, lithiophorite, manganite, and hausmannite in general.
In another aspect, a mixture of discrete granules of the phosphorus source and discrete granules of the oxide of manganese are present in a premix which can be mixed with the soil to achieve the above-described levels of phosphorus and manganese oxide.
As used herein, xe2x80x9cdiscretexe2x80x9d and xe2x80x9cphysically separate from one anotherxe2x80x9d is intended to mean that the granules do not include both the phosphorus source and the oxide of manganese in the same granule.
Alternately, the components can be mixed with the soil individually in the concentrations described previously, preferably with the phosphorus source being mixed with the soil first followed by mixing of the manganese oxide with the resulting phosphorus/soil mixture.
In yet another embodiment, the phosphorus source and oxide of manganese can be mixed so as to form a substantially homogeneous mixture. The resulting mixture can then be granulated by using known granulation processes, and the granulated product can be used according to the inventive methods.
Regardless of the embodiment, after mixing with source of phosphorus and oxide of manganese with the soil, the pH of the soil is preferably adjusted (if necessary) to a level of at least about 7.0, and preferably to a level of from about 7.0-8.0. Prior to pH adjustment, the soil will generally be acidic, thus requiring the use of an alkaline material such as CaO, CaCO3, MgCO3, quick lime, limestone, and Ag-lime. The pH adjustment should be carried out from about 20-28 hours, and preferably about 24 hours, after the phosphorus source and oxide of manganese are mixed with the soil. It is preferred that the pH of the soil be monitored at regular intervals (e.g., every year) after treatment so that the pH can be maintained in the desired range. Finally, those skilled in the art will appreciate that soil treatment according to the invention can be carried out in situ, or the soil can be excavated and moved to another location for treatment and then returned to its original location.
It is believed that the inventive methods and compositions decrease the bioavailability of the metal contaminant by forming essentially irreversibly adsorbed metals and by causing the metal to react with the phosphorus source to form insoluble metal phosphates (e.g., lead phosphate minerals or pyromorphites), thus rendering the metal contaminant non-bioavailable. Thus, the decontaminated soil will include a total non-naturally occurring phosphate content (as used in this context, the phosphate fraction of the resulting metal phosphates and the phosphate fraction with metals adsorbed thereon) of at least about 0.1% by weight phosphate, preferably at least about 0.5% by weight phosphate, and more preferably from about 1-5% by weight phosphate, based upon the total weight of the soil taken as 100% by weight.
The inventive methods and compositions are particularly useful for reducing the bioavailability of Group IIB metals (e.g., lead, zinc, cadmium) and Group VIII metals (e.g., iron, nickel). Thus, about 24 hours after the phosphorus source and oxide of manganese are mixed with the contaminated soil, the metal bioavailability in the soil is reduced by at least about 20%, and preferably at least about 40%, with the metal bioavailability being determined by the stomach phase of the Physiologically Based Extractions Test (PBET) as herein defined. Or, as determined by the Toxicity Characteristic Leaching Procedure (TCLP) as herein defined, the leachability of lead is less than about 5 mg/L, and preferably less than about 2 mg/L, about 24 hours after treatment.
Furthermore, 90 days after treatment, the bioavailability of the metal will increase by less than about 10%, and preferably less than about 5%, above the bioavailability about 24 hours after mixing. Finally, even after the growing of plants in the treated soil, the bioavailability of the contaminant metals will remain substantially unchanged. That is, if a plant is allowed to grow for about 8 weeks in the treated soil, the bioavailability of metal in the soil will increase by less than about 10%, and preferably less than about 5%, during the plant growth.